Removing material from a substrate to form microscopic or nanoscopic structures is referred to as micromachining, milling, or etching. Lasers beams and charged particle beams are two particular technologies used for micromachining. Each has advantages and limitations in various applications.
Laser systems use several different mechanisms for micromachining. In some processes, the laser is used to supply heat to a substrate to induce a chemical reaction. The reaction occurs only in the heated areas. The heat tends to diffuse to an area larger than the laser beam spot, making the resolution of the process poorer than the laser spot size and causing concomitant thermal damage to nearby structures. Another mechanism used in laser micromachining is photochemical etching, in which the laser energy is absorbed by individual atoms or molecules (particles) of the substrate, exciting them into a state in which they can chemically react with an etchant. Photochemical etching is limited to materials that are photochemically active. Another mechanism used in laser machining is laser ablation, in which energy supplied rapidly to a small volume causes atoms to be explosively expelled from the substrate. Laser ablation using an ultrashort pulsed laser (UPL) is described, for example, in U.S. Re. 37,585 to Mourou for “Method for Controlling Configuration of Laser Induced Breakdown and Ablation.” UPL ablation (UPLA) overcomes some of the limitations of the processes described above.
Charged particle beams include ion beams and electron beams. Ions in a focused beam typically have sufficient momentum to micromachine by physically ejecting material from a surface. Because electrons are much lighter than ions, electron beams are typically limited to removing material by inducing a chemical reaction between an etchant vapor and the substrate. Ions beams typically are generated from a liquid metal ion source or by a plasma ion source. The spot size of a charged particle beam depends on many factors, including the type of particles and the current in the beam. A beam with low current can typically be focused to a smaller spot and therefore produce a smaller structure than a beam with high current, but a low current beam takes longer to micromachine a structure than a high current beam.
Lasers are typically capable of supplying energy to a substrate at a much higher rate than charged particle beams, and so lasers typically have much higher material removal rates than charged particle beams. FIG. 1 is a schematic illustration of a prior art laser ablating a surface. When a high power pulsed laser 102 producing beam 103 is focused onto a target material 104 and the laser fluence exceeds the ablation threshold of the material, chemical bonds in the target material are broken and the material is fractured into energetic fragments, typically a mixture of neutral atoms, ions, clusters, and nano- and micro-particles creating a plasma plume 106 above the material surface. Since the material leaves the reaction zone as an energetic plasma, gas, and solid debris mixture, the ablation process resembles explosive evaporation of the material which propels material fragments up and away from the point where the laser is focused. As the plasma cools, much of the solid debris 108 is redeposited on the workpiece surface, thus reducing the quality of the cut and decreasing the cutting efficiency since the debris must be removed again before the beam interacts with the workpiece surface.
Various techniques are known to minimize undesirable redeposition during laser ablation. For example, it is known to use an inert gas stream to cool the ablation site as described by Gua, Hongping et al. “Study of Gas-Stream Assisted Laser Ablation of Copper,” 218 THIN SOLID FILMS, 274-276 (1992). U.S. Pat. No. 5,496,985 to Foltz et al. for “Laser Ablation Nozzle,” describes the use of gas or fluid jets to remove ejected material from the vicinity of the cut to prevent redeposition in that area. Robinson, G. M. et al. “Femtosecond Laser Micromachining of Aluminum Surfaces Under Controlled Gas Atmospheres,” J. Mater. Eng. & Perf., Vol. 15(2), 155-160 (April 2006) describe the use of an inert gas as an inert gas shield. Such secondary techniques are often not completely effective, and add significant complexity to the laser ablation system while decreasing cutting efficiency.
U.S. Pat. Pub. No. 2008/0241425 by Li et al. for “System and Method to Reduce Redeposition of Ablated Material,” filed Oct. 2, 2008, describes a system where laser ablation is performed in a vacuum. According to Li, most laser ablation is performed in air (at normal atmospheric pressure) for low cost and convenience. Li teaches lowering the pressure in the sample chamber so that the ablated material will travel farther from the milling site before it loses a significant amount of kinetic energy and redeposits onto the surface. Unfortunately, even using the method described by Li, the material still redeposits onto the surface, some of it just travels farther away than it would at higher chamber pressure. Further, the low pressure system described by Li makes it more likely that the debris material will deposit onto various system components, such as lenses and pole pieces, as described below.
Like lasers, charged particle beam systems also have a problem with material redeposition. In order to prevent significant redeposition and increase the material removal rate, charged particle beam systems often make use of gas-assisted etching (GAE). In GAE, an etching gas (also referred to as a precursor gas) is directed at the material surface so that a monolayer of gas particles (molecules or atoms depending on the type of gas) is adsorbed onto the material surface. Irradiation of the material surface by a charged particle beam leads to the dissociation of the adsorbate, producing reactive fragments that react with the sample material to form volatile products that can be pumped away. A significant factor in the rate of material removal is the rate at which gas particles are adsorbed on the surface. If a charged particle beam, such as a focused ion beam (FIB), dwells too long in one location, all adsorbates will be dissociated or desorbed and the beam will begin to remove material by sputtering, with the resulting problem of material redeposition. While milling, the ion beam is typically scanned repeatedly over a rectangle in a raster pattern. As the beam completes a scan, the beam is typically delayed for a significant amount of time before beginning the next scan to provide time for additional gas particles to adsorb onto the surface before beginning a new raster. This increases processing time. High concentrations (high gas pressures) of the precursor gas are not generally helpful because only a relatively small number of particles, forming a monolayer on the surface, adsorb onto the material surface at a time.
A similar gas-assisted etching process employing long pulse and continuous wave lasers is known as photochemical etching. Photochemical Laser Etching (PLE) involves directing a beam at the workpiece surface with an energy level below the ablation threshold of the material being processed while the workpiece surface is exposed to a precursor gas. Instead of removing the material by the very rapid process of thermal ablation described above, the laser only provides energy to the adsorbed gas particles causing the formation of a volatile compound that chemically etches the surface. While PLE does prevent redeposition artifacts, the material removal rate using this process is a fraction of the rate using thermal ablation.
Laser-assisted chemical etching (LCE) is another well known technique combining nanosecond lasers and reactive gasses to etch certain substrates, such as silicon, in the presence of a high-pressure gas, such as chlorine. Such a process is described, for example, by Daniel Ehrlich et al., Laser Etching for Flip-Chip De-Bug and Inverse Stereolithography for MEMS, SOLID STATE TECH., Jun. 2001, pp. 145-150 (“Ehrlich”). In the process described by Ehrlich, however, the laser is not used to ablate the sample surface; rather the laser is used to heat the silicon until it becomes molten. The molten silicon then reacts with the chlorine gas to etch the silicon. According to Ehrlich, as the chlorine reactant pressure is increased, etching (directly proportional to the reaction flux) increases linearly for a time then saturates as the nonlinear effect of gas diffusion limits long-range transfer of reactant gas due to the formation of a depleted “boundary layer.” As a result, etching rates (without redeposition) can only be increased to a relatively low rate when compared to the removal rates that are possible using thermal ablation.
What is needed is an improved method for laser processing that prevents material redeposition during laser ablation but allows material to be removed at a high rate.